This invention relates to a process for recovering organic material, primarily liquid hydrocarbons, from solids containing organic material and is particularly concerned with a process for extracting hydrocarbons from oil shale that has been heated to temperatures below those encounted in conventional oil shale retorting processes.
Because of a dwindling supply of petroleum liquids from underground reservoirs, attention has recently been focused on the recovery of hydrocarbon liquids and gases from solids such as oil shale, coal, industrial and municipal solid wastes and the like. Work by both governmental agencies and private industry has demonstrated that the organic material in such solids can be converted with varying degrees of difficulty into hydrocarbonaceous fluids such as combustible gases, motor fuels, heating and fuel oils, and various by-products which are of value in chemical and petrochemical industries. In general, the more attractive of the recovery techniques previously proposed involve the heat treatment of such solids in a manner sufficient to pyrolyze or otherwise decompose the organic material into the above-mentioned hydrocarbonaceous products.
Oil shale is considered to be one of the best candidates of all hydrocarbon-containing materials for processing in a retorting or pyrolysis scheme since it is a mixture of a minor amount of solid organic matter called kerogen and a major amount of mineral matter. The organic matter or kerogen is a polymer which is virtually insoluble in organic solvents. Because of this insolubility, it has been difficult in the past to extract the kerogen from the oil shale and therefore the application of heat via pyrolysis or retorting has had to be used The retorting process is carried out at relatively high temperatures, normally between about 900.degree. F. and about 1000.degree. F., in order to cause the solid organic matter to undergo destructive pyrolysis and simultaneous conversion into various hydrocarbonaceous products. In general, only about 75 percent of the organic material in the oil shale is recovered as fluid products. The other 25 percent of the organic material is left as coke or a carbon-rich residue on the spent shale particles. Although this residue can be used as a fuel, it is advantageous to convert more of the organic material into recoverable liquid hydrocarbons. Furthermore, the spent shale particles contain over 95 percent by weight of inert materials and are cumbersome to burn as a fuel. Another disadvantage of retorting is that it does not offer the flexibility to change the liquid yield by either decreasing or increasing the amount of carbon-rich residue remaining on the solids.
An alternative approach to retorting is to heat the oil shale in the presence of a liquid solvent to extract the organic material from the rock matrix as is taught in U.S. Pat. No. 1,327,572. In the disclosed process, ground raw shale is slurried with a heavy oil and heated sufficiently to liquify the hydrocarbon in the rock. Processes such as this must normally be carried out at relatively high pressures and temperatures with large solvent-to-shale ratios in order to obtain liquid yields comparable to those produced in conventional retorting processes. Operation at high pressure and temperature, however, has major disadvantages. The equipment utilized in handling solids under pressure is very complex and the solvent tends to degrade at high temperature. Furthermore, it has been found that when raw shale is mixed with hot solvent or previously heated shale is mixed with solvent at any temperature, the shale has a tendency to disintegrate into fines that are difficult to separate from the solvent and extracted liquids.
Accordingly, it is one of the objects of the present invention to provide a process for increasing the yield of hydrocarbon liquids from the processing of solids containing organic material. It is another object to provide such a process which can be operated at relatively low pressures and temperatures. It is a further object of the invention to provide a process which can be operated to avoid degradation of the extraction solvent and excessive distintegration of the solids undergoing extraction. It is yet a further object to provide a process in which the liquid yields can be controlled as desired by leaving more or less organic material in the spent solids. These and other advantages will become more apparent in view of the following description of the invention.